Inorganic thermoset resins and methods of making thereof

ABSTRACT

In a first aspect, the present disclosure provides a method for making an inorganic thermoset resin, the method comprising:
     (a) mixing SiO 2 , H 2 O and a metallic hydroxide for generating an alkaline aqueous solution with pH from 10 to 14 comprising a metallic silicate, wherein said metallic hydroxide generates a first metallic oxide in the aqueous solution,   (b) adding aluminum oxide (Al 2 O 3 ) and silicon oxide (SiO 2 ) to the alkaline aqueous solution comprising a metallic silicate generated in step (a) and   (c) adding halloysite nanotubes (Al 2 Si 2 O 5 (OH) 4 ) to the solution generated in step (b).   

     The present disclosure further provides an inorganic thermoset resin obtainable by the method as defined in the first aspect of the disclosure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of European Application Serial Number EP15382423.0, filed Aug. 10, 2015, which is herein incorporated byreference in its entirety.

FIELD

The present disclosure relates to methods for making fire resistantinorganic thermoset resins. Particularly, this disclosure relates to amethod for making a fire resistant inorganic thermoset resin comprisingadding metal oxides and halloysite nanotubes to an alkaline silicatesolution.

BACKGROUND

Previous solutions to make inorganic thermoset resins included, forexample, contacting potassium silicate solution, water, Al₂O₃, ZrO₂ andkaolin. ZrO₂ and kaolin are common fillers in geopolymeric resins. A gelis formed that finally forms the inorganic thermoset resin. Then,metakaolin in powder form is added to the thermoset resin to avoidshrinkage during the resin curing. Dissolution of metakaolin powder isnot a straightforward step, since this powder has low solubility.Metakaolin needs to be dissolved for a predetermined period of time.This is a time consuming and expensive step (metakaolin needs to befinely dispersed in the alkaline solution and that involves a complexprocess using a special mixing apparatus). Moreover, since metakaolinhas to be added in a certain % weight of the resin, the addition ofmetakaolin considerably increases the weight of the resin.

There is a need in the art for an improved method to make fire resistantinorganic thermoset resins which eliminates the step of addition of theanti-shrinkage filler metakaolin, while at the same time provides a fireresistant inorganic thermoset resin which does not shrink during theresin curing.

SUMMARY

In a first aspect, the present disclosure provides a method for makingan inorganic thermoset resin, the method comprising:

(a) mixing SiO₂, H₂O and a metallic hydroxide for generating an alkalineaqueous solution with pH from 10 to 14 comprising a metallic silicate,wherein said metallic hydroxide generates a first metallic oxide in theaqueous solution,

(b) adding aluminum oxide (Al₂O₃) and silicon oxide (SiO₂) to thealkaline aqueous solution comprising a metallic silicate generated instep (a) and

(c) adding halloysite nanotubes (Al₂Si₂O₅(OH)₄) to the solutiongenerated in step (b).

The present disclosure further provides an inorganic thermoset resinobtainable by the method as defined in the first aspect of thedisclosure.

DETAILED DESCRIPTION

A first aspect of the present disclosure is a method for making aninorganic thermoset resin, the method comprising:

(a) mixing SiO₂, H₂O and a metallic hydroxide for generating an alkalineaqueous solution with pH from 10 to 14 comprising a metallic silicate,wherein said metallic hydroxide generates a first metallic oxide in theaqueous solution,

(b) adding aluminum oxide (Al₂O₃) and silicon oxide (SiO₂) to thealkaline aqueous solution comprising a metallic silicate generated instep (a) and

(c) adding halloysite nanotubes (Al₂Si₂O₅(OH)₄) to the solutiongenerated in step (b).

Halloysite is defined as an aluminosilicate clay mineral mined fromnatural geological deposits that is chemically identical to kaolin, butdiffers in its morphology by possessing a unique crystal structure whichcan, under proper geological conditions, facilitate the formation ofhollow nanotubes rather than only a stacked plate-like structure asobserved in kaolin. In the present disclosure, Halloysite nanotubes arereferred interchangeably as halloysite nanotubes or HNT.

Halloysite nanotubes are abundant and cheap.

The present disclosure provides a simplified method to produce inorganicthermoset resins.

The present disclosure eliminates the need for metakaolin (antishrinkage agent). Instead of kaolin, halloysite nanotubes are used.

Halloysite nanotubes have the same chemical formula as kaolin but have adifferent structural morphology. However, the mechanism of the reactionthat liberates free —Si—O—Al-species is the same as described below forkaolin but, in addition, halloysite nanotubes prevent the geopolymerresin from cracking during and after polymerization (curing) of theresin, eliminating the need for metakaolin. Thus, methods of the presentdisclosure:

-   -   Offer a simplified manufacturing process since the metakaolin        filler addition, which was essential to avoid shrinkage during        the curing of the resin, and which involved an expensive and        time consuming process, is no longer needed. The filler addition        is done in just one step leading to oligo-sialate formation.    -   Halloysite nanotubes act as a nanoscale reinforcement filler        material effectively forming a nanocomposite material. This        leads to improved mechanical properties.    -   Reduces the weight of the resin, since metakaolin is not needed.

Halloysite nanotubes have a double functionality in the resin synthesis:(1) is part of the composition of the resin forming Si—O—Al speciesnecessary for the proper formation of the geopolymeric resin; and (2) italso serves as filler eliminating the addition of metakaolin, which istime consuming, expensive and increases the weight of the geopolymerresin prior art.

Methods of the present disclosure eliminate the step of addingmetakaolin and makes the production of the inorganic thermoset resineasier than existing solutions.

ZrO₂ and kaolin are common fillers in geopolymeric resins. Additionalmineral fillers such as metakaolin were added to inorganic thermosetresins to avoid shrinkage after curing the resin. The addition ofmetakaolin to the inorganic thermoset resins was not a simple process.Metakaolin is a powder that needed to be finely dispersed in order to beincorporated into the solution. This process was done by using aparticular mixing apparatus and was very time consuming.

Moreover, the addition of metakaolin contributes to increasing theweight of the resin prepared.

Therefore, the synthesis should be simplified in order to make itsproduction feasible at industrial scale.

The present disclosure is directed to the addition of halloysitenanotubes as nano-scale additive for inorganic polymer resins as well asa filler of the inorganic polymer thermoset resins.

In a second aspect of the present disclosure, in step (a) the metallicsilicate is selected from the group consisting of silicate of an alkalimetal, silicate of an alkaline earth metal and silicate of a transitionmetal and mixtures thereof.

In the second aspect of the present disclosure, the metal is selectedfrom the group consisting of Na, K, Li, Ca, Mg and Fe.

In a third aspect of the present disclosure, in step (a) theconcentration of the metallic silicate is between about 3 M and about 4M.

In a fourth aspect of the present disclosure, in step (b) the firstmetallic oxide/SiO₂ molar ratio is between about 0.08 and about 0.4.

In a fifth aspect of the present disclosure, in step (b) the firstmetallic oxide/Al₂O₃ molar ratio is between about 0.6 and about 15.

In a sixth aspect of the present disclosure, in step (b) the SiO₂/Al₂O₃molar ratio is between about 3.5 and about 100.

In a seventh aspect of the present disclosure, a second metallic oxideis added to the solution generated in step (b), wherein the secondmetallic oxide is an oxide of a transition metal.

In the seventh aspect of the present disclosure, the transition metal isZr or Ti.

In the seventh aspect of the present disclosure, the second metallicoxide is added at a concentration of between about 1% and about 10% byweight with respect to the total weight of the aqueous solution.

In an eighth aspect of the present disclosure, in step (c) halloysitenanotubes (Al₂Si₂O₅(OH)₄) are added at a concentration of between about2% and about 20% by weight with respect to the total weight of theaqueous solution.

In a ninth aspect, the present disclosure further provides an inorganicthermoset resin formed by the method as defined in the first aspect ofthe disclosure.

The curing of the inorganic thermoset resin according to the seventhaspect of the disclosure is carried out by heating at a temperature ofbetween about 60° C. and about 80° C. for between about 1.5 hours andabout 3 hours. The curing may be also carried out at ambient temperaturefor several hours, for example, about 24 hours.

The inorganic thermoset resin according to the seventh aspect of thedisclosure has many applications in several fields. It could be used inaerospace field for aircraft and space vehicles. It could also be usedin trains and automobiles, vessels and other transportation vehicles andby composite manufacturers. In fact, the resin could be incorporated inany industrially manufactured products or goods. The inorganic thermosetresin may be used as a resin in state-of-the-art fiber-reinforcedinorganic polymer resin (FRIP) composites.

The outstanding fire resistance of the inorganic thermoset resinaccording to, for example, the seventh aspect of the disclosure up to1000° C., makes it a very sought after product for applications wherethere are stringent fire resistance requirements. For example, inaircraft interiors.

The inorganic thermoset resin according to, for example, the seventhaspect of the disclosure can be used in any type of light weight fiberreinforced composites subjected to stringent fire resistancerequirements. Although its application is certainly not restricted toaircraft applications, one high value application is in natural fiberreinforced composites in sandwich panels for aircraft interiors.

The inorganic thermoset resin according to the seventh aspect of thedisclosure is obtained from natural nanoscale minerals and industry byproducts that are safe to produce and do not require specialized safetyequipment, do not produce harmful chemical residues and are naturallyfire resistant. The outstanding fire resistant properties of theinorganic thermoset resin of the disclosure are the result of its mainingredients being fire resistant themselves. This eliminates the need toapply or employ fire resistant additives in order to make the systemfire resistant.

EXAMPLES Example 1 Preparation of the Inorganic Thermoset Resin

Methods of the present disclosure comprise preparing a stable alkalineaqueous solution. Solutions comprise a Potassium Silicate solution(K₂SiO₃) using the starting components SiO₂, H₂O and KOH. K₂O is formedin the reaction from KOH.

Moreover, NaOH or any other suitable metallic hydroxide could be used togenerate the corresponding metallic silicate as the alkaline silicatebased medium which is necessary to obtain the geopolymeric resin.

The starting components are used as follows:

-   -   Molar ratio K₂O/SiO₂=1    -   50% by weight is H₂O/50% by weight is solid (KOH and SiO₂).

This solution reaches about pH=13.

(SiO₂K₂O. H₂O═K₂SiO₃. H₂O)

Then, water was added until a 3.5 M silicate K₂SiO₃ aqueous solution wasgenerated.

Then, Aluminum oxide (Al₂O₃) and silicon oxide (SiO₂) were added andmixed for 5 minutes.

In this step, the following molar ratios were used:

-   -   K₂O/SiO₂=0.16    -   SiO₂/Al₂O₃=16    -   K₂O/Al₂O₃=2.61

Then, Zirconium oxide (ZrO₂) was added at a concentration of 4% byweight with respect to the total weight of the aqueous solution andmixed for 5 minutes.

Halloysite nanotubes (Al₂Si₂O₅(OH)₄) in 10% by weight with respect tothe total weight of the aqueous solution were then added and mixed for 1hour.

All of these processes were carried out at ambient temperature.

The inorganic thermoset resin was then cured at 80° C. for 2 hours.

Example 2 Boiling Water Tests

The Boiling water test (BWT) is a test procedure known by the skilledperson that determines whether a thermoset inorganic resin has undergonea correct polymerization and, therefore, if it has properly cured. Theappropriate polymerization of the resin is crucial for the applicabilityof the resin so it could also be considered as a screening test beforefine tuning any process development for inorganic thermoset resins.

The test consists of introducing small pieces of the cured resin inboiled water during 20 minutes. There are two possible scenarios:

-   -   The samples of the cured resin do not suffer deformation or        degradation after the boiling water tests, indicating the        correct polymerization of the resin. In this case, the resin        passes the Boiling water test, showing that the resin has        properly cured.    -   The samples of cured resin suffer deformation or disintegration        after (or before) the 20 min boiling water test, indicating that        the resin has not polymerized. In this case the resin fails the        BWT, showing that the resin has not properly cured.

To provide an accurate estimation of whether the samples sufferdeformation and/or disintegration during such test, the diameter of thesmall pieces of cured resin samples subjected to the test are measuredbefore and after the BWTs. This allows measuring any possible variationthat the diameter of the cured samples could suffer at a millimeterscale.

Boiling Water Test Results:

Samples of the inorganic thermoset resin of the disclosure weresubjected to the BWTs in order to assess if the resin had properlypolymerized using. The resin successfully passed the BWTs since therewas diameter resilience and no variation/disintegration of the sampleswas observed after immersing the samples in boiling water for 20 min.This indicates a correct polymerization/curing of the resin.

Example 3 Fire Resistance

The inorganic thermoset resin's fire resistance was tested by subjecting3 sandwich panels comprising outer skins made of flax fibers+inorganicthermoset resin of the disclosure, and a polyetherimide (PEI) foam as acore material to OSU test (Ohio State University test), which is themost stringent test to comply with fire requirements.

The OSU test is a method used to determine the Heat Release Rate fromcabin materials exposed to radiant Heat (FAR 25, Appendix F part IV).

Summary of OSU Test

The specimen to be tested is injected into an environmental chamberthrough which a constant flow of air passes. The specimen's exposure isdetermined by a radiant heat source adjusted to produce the desiredtotal heat flux on the specimen of 3.5 Watts/cm², using a calibratedcalorimeter.

The specimen is tested so that the exposed surface is vertical.Combustion is initiated by piloted ignition. The combustion productsleaving the chamber are monitored in order to calculate the release rateof heat, which must be below 65 kW/m².

Table 1 shows that the panels prepared with inorganic thermoset resin ofthe disclosure successfully passed the OSU tests.

The resin has excellent fire resistance, which makes it suitable formany applications, including, but not limited to, aircraft interiors.

TABLE 1 OSU tests results of the panels prepared using the inorganicthermoset resin of the disclosure. Total heat Heat released after thefirst Panel released (kW/m²) two minutes (kW * min/m²) Panel 1 35.43 (a72 s) 22.45 Panel 2 30.15 (a 56 s) 28.73 Panel 3 36.30 (a 67 s) 31.99Values obtained 33.96 27.72 Threshold value <65 <65

1. A method for making an inorganic thermoset resin, the methodcomprising: mixing SiO₂, H₂O and a metallic hydroxide to form analkaline aqueous solution with pH between about 10 and about 14comprising a metallic silicate, wherein said metallic hydroxidegenerates a first metallic oxide in the aqueous solution; addingaluminum oxide (Al₂O₃) and silicon oxide (SiO₂) to the alkaline aqueoussolution comprising a metallic silicate; and adding halloysite nanotubes(Al₂Si₂O₅(OH)₄) to the alkaline aqueous solution.
 2. The methodaccording to claim 1, wherein the metallic silicate is selected from thegroup consisting of silicate of an alkali metal, silicate of an alkalineearth metal, silicate of a transition metal, and mixtures thereof. 3.The method according to claim 2, wherein said metal is selected from thegroup consisting of Na, K, Li, Ca, Mg and Fe.
 4. The method according toclaim 1, wherein the concentration of said metallic silicate is betweenabout 3M and about 4 M.
 5. The method according to claim 1, wherein thefirst metallic oxide/SiO₂ molar ratio is between about 0.08 and about0.4.
 6. The method according to claim 1, wherein in the first metallicoxide/Al₂O₃ molar ratio is between about 0.6 and about
 15. 7. The methodaccording to claim 1, wherein in the SiO₂/Al₂O₃ molar ratio is betweenabout 3.5 and about
 100. 8. The method according to claim 1, wherein asecond metallic oxide is added to the alkaline aqueous solution, whereinsaid second metallic oxide is an oxide of a transition metal.
 9. Themethod according to claim 8, wherein said transition metal is Zr or Ti.10. The method according to any one of claim 8, wherein said secondmetallic oxide is added at a concentration of between about 1% and about10% by weight with respect to the total weight of the aqueous solution.5
 11. The method according to claim 8, further comprising curing theresin at a temperature of between about 60° C. and about 80° C.
 12. Themethod according to claim 11, wherein curing is performed for betweenabout 1.5 hours and about 3 hours.
 13. The method according to claim 8,further comprising curing the resin at ambient temperature for about 24hours.
 14. The method according to claim 1, wherein halloysite nanotubes(Al₂Si₂O₅(OH)₄) are added at a concentration of between about 2% andabout 20% by weight with respect to the total weight of the aqueoussolution.
 15. The method according to claim 1, further comprising curingthe resin at a temperature of between about 60° C. and about 80° C. 16.The method according to claim 15, wherein curing is performed forbetween about 1.5 hours and about 3 hours.
 17. The method according toclaim 1, further comprising curing the resin at ambient temperature forabout 24 hours.
 18. An inorganic thermoset resin formed by the method ofclaim
 1. 19. The inorganic thermoset resin of claim 18, wherein theinorganic thermoset resin comprises a heat release value of less than 65(kW*min/m²)